The invention relates to systems and methods for starting synchronous motors so as to protect the amortisseur or cage winding from thermal damage including when starting synchronous motors at voltage levels reduced from the full rated motor voltage.
Polyphase synchronous motors usually have the AC excited winding on the stator core and poles and field structure on the rotor assembly. During normal operation, the rotor assembly rotates at synchronous speed which is a function of excitation frequency and the number of pole pairs. The AC winding produces a magnetic field, i.e., mmf, revolving at the synchronous frequency in the air gap between the rotor and stator assemblies. A flux that is induced in the physical rotating field structure "locks" with this rotating magneto motive force causing the rotating field structure to turn at synchronous speed and to produce "synchronous torque". However, this normal operation does not occur until the rotating field structure has been brought up to synchronous speed.
Synchronous motors require special, i.e., artificial, starting arrangements to bring them up to synchronous speed, since without such arrangements, these motors have no starting torque.
For start up and damping purposes, a squirrel cage winding, or amortisseur winding, is commonly installed adjacent to the pole faces of the motor assembly. This cage winding provides for start up in the fashion of an induction motor. The rotating magneto motive force produced by the stator winding cuts the conductors of the cage or amortisseur winding and generates voltages and currents in these bars to produce rotor magnetic poles. The rotor voltage is determined by the slip, i.e., the difference between the speed of the rotating magnetic field produced by the starter and the motor, i.e., rotor speed. This start up arrangement provides starting torque that brings the motor close to synchronous speed.
Generally, the inherent resistance of cage windings prevents the motor from running without slip and thus prevents it from attaining synchronous speed. Therefore, supplemental starting means are used to pull the motor into synchronous speed. DC excitation is applied to separate rotor field windings to create constant polarity poles in the rotor. DC excitation, however, does not provide useful torque until the rotor approaches synchronous speed. During the presence of substantial slip, dc excitation produces positive and negative torque during successive half cycles. This can result in undesirable oscillatory torque components and a negative or braking torque that reduces the acceleration torque otherwise produced. These undesirable characteristics diminish as slip is reduced. Therefore, dc excitation is usually not applied until the slip is very low, for example, 5%, equivalent to 95% synchronous speed.
The amortisseur or cage winding of a synchronous motor is essentially operative only during start up, i.e., until field winding excitation and pull in of the motor to synchronous speed. The winding must be accomodated in a limited space in the motor. It is therefore usually made of lighter material than the cage winding of an induction motor and is susceptible to overheating and thermal damage. The magnitude of the heat is related to the product of the squared value of rotor current and of the time duration of the current.
The rotor current, and thus particularly its squared value, is exceedingly high when the motor is energized while at standstill, i.e., a locked rotor condition. Thermal damage quickly results if the stalled motor is energized in excess of the very brief allowable stall time of the motor.
During start up, as the motor speed increases from standstill toward synchronous speed, cage winding heating diminishes because of the decreasing frequency of the rotor current and the increased air circulation from the fan effect of the rotor. Nevertheless, thermal damage can occur if the motor runs below synchronous speed for running times exceeding those of an acceleration schedule based on the motor characteristics.
Similarly, thermal damage can occur if the motor runs for a substantial time out of synchronism without excitation. The motor then runs as an induction motor with slip producing continuous cage current to develop torque, but also heat.
Motor controllers are therefore preferably designed to "trip", i.e., open the main contactor of the motor to remove ac excitation before permanent damage occurs under these conditions. The time duration of motor energization is therefore a primary parameter in start up protection.
Exemplary are synchronous motor controls of General Electric Company described in "Instructions Synchronous Motor Control With IC 3655A105 Solid State Starting and Protection Module", GEH-3133D. These controls integrate signals derived from the motor field winding circuit having a value representative of motor slip, an inverse function of the percentage of synchronous speed. They remove ac motor energization, i.e., "trip" the motor, when the integrated value attains a predetermined magnitude representative of allowable stall time. This arrangement provides some protection against the actual motor running time exceeding allowable stall time of the motor. However, protection is based on approximations as opposed to being readily adaptable to the specific characteristics of the motor and may not permit the motor to run for the maximum allowable running time prior to tripping.
Squirrel cage protection has also been adversely affected during reduced voltage start operation, specifically by nuisance tripping, i.e., the unnecessary removal of ac energization during start up. Synchronous motors are frequently started with ac energization reduced below the full rated voltage. Energization at full rated voltage initially produces an extremely large inrush current which can produce troublesome voltage fluctuations on the power supply lines. Start up with a reduced voltage provides a proportionate reduction of inrush current and thus reduces line fluctuations. However, with reduced voltage starting, starting torque is substantially reduced since it is proportional to the square of the energization voltage. This reduces motor acceleration and increases the time required to attain synchronous speed. Therefore, start protection controls whose acceleration schedule is based on full rated voltage operation may unnecessarily trip a motor energized with reduced voltage at a time before the motor attains synchronous speed.
After initial start up at reduced voltage, the ac energization is increased to full rated voltage to assure proper synchronous speed operation. Full rated voltage is frequently applied during start up after some finite period of reduced voltage excitation. Thus, any measures taken to minimize nuisance tripping should not adversely affect start protection accorded during the subsequent application of full rated voltage.